1. Field of the Invention
The present invention relates to variable displacement compressors and methods for controlling the same.
2. Description of the Related Art
A typical variable displacement compressor used in automobiles includes a crank chamber that is provided within a housing. A drive shaft is rotatably supported in the crank chamber. Part of the housing includes a cylinder block through which a plurality of cylinder bores extend. A piston is reciprocally accommodated in each cylinder bore. A cam plate is provided on the drive shaft. The cam plate rotates integrally with the drive shaft and is supported so as to incline with respect to the drive shaft during rotation. Each piston is coupled to the cam plate. The stroke of the piston during reciprocation within the associated cyliner bore is determined by the inclination of the cam plate.
The inclination of the cam plate is controlled by adjusting the pressure in either the crank chamber or the suction chamber. In other words, the difference between the pressures acting on both ends of each piston may be changed by altering the pressure in either one of the crank chamber and the suction chamber. This changes the inclination of the cam plate and varies the compressor displacement. Furthermore, an electromagnetic displacement control valve is arranged in a passage extending between the crank chamber and the suction chamber or between the discharge pressure zone and the suction pressure zone to adjust the opened area of the passage. The control valve includes a pressure sensinq mechanism, which transmits fluctuations of the suction pressure to a valve body, and a solenoid, which is used to alter the load applied on the valve body in accordance with the electric current flowing therethrough to change the suction pressure.
The opened area of the passage is determined in accordance with the suction pressure fluctuation, temperatures in an external refrigerant circuit, temperature at various locations within the vehicle, and various actuating information. This alters the flow rate of the high-pressure refrigerant gas supplied to the crank chamber from the discharge pressure zone. The difference between the pressure in the crank chamber and the suction pressure alters the inclination of the cam plate. Thus, the compressor displacement is controlled in accordance with various conditions.
In addition to the inclined angle of the can plate, the compressor displacement is affected by the rotating speed of the cam plate. The cam plate is rotated between a high speed range and a low speed range. When the rotating speed of the cam plate is high, the compressor displacement increases and when the rotating speed of the cam plate is low, the compressor displacement decreases. The rotating speed of the cam plate is hereafter referred to as the compressor speed.
With high performance engines having high rotating speed ranges, it is necessary for the compressor to operate at high rotating speeds. When the compressor displacement becomes maximum with the drive shaft rotating at high speed, the load applied to the compressor is extremely large. The drive shaft slides against lip seale, which prevent refrigerant gas from leaking out of the crank chamber at high rotating speeds. Furthermore, high rotating speeds produce a high compression reaction load acting on the bearings supporting the drive shaft. As a result, parts may overheat and the lubrication of parts that slide against other parts may become insufficient. This may degrade the durability of the compressor.
In compressors that control the inclination of the cam plate with the pressure in the crank chamber, the refrigerant gas from the external refrigerant circuit is not supplied to the suction chamber by way of the crank chamber. In other words, refrigerant gas does not flow between the suction pressure zone and the discharge pressure zone. When the cam plate is rotating at high speed, the amount of hot high-pressure blow-by gas that passes through slight gaps defined between the pistons and the cylinder block increases. This may increase the temperature and pressure in the crank chamber, thus causing the lubrication and cooling of the sliding parts to be inadequate.